Greetings, I am Dr. Pat. I have worked with birds for many years. I will do my best to help you.
I am sorry no other expert has taken your question. We all come online at different times, I have just logged in and saw that you have not been answered. I hope I can still be of assistance.
There are seve
ral avenues to explore in the failure of parasite treatment.
1. Proper ID of the parasite: you have collected the organisms and they are for certain lice?
2. Resistance of the parasite to the product. It does happen.
3. Integrity of the product: was the ivermectin fresh, at proper dosage and application?
4. Did EVERY bird receive treatment? missing even one with cause another outbreak.
The entire environment needs to be considered. The coop, bedding, perches, etc. need to be very clean and fresh. Grass hay at about 6" deep is the best sustrate, kept clean and dry.
If the birds are overcrowded, they will continue to infect each other.
They need to be on a perfect diet. They need at least 12 hr darkness at night, to diminish the louse reproductive opportunities.
I would recommend "starting over": Complete clean of the coop, run, etc; samples of the lice to be identified by an expert; reapplication of anti-lice preparation supplied by a veterinarian; review of husbandry and care practices.
There are other products, some which can be added to the dust-bath area.
I would strongly urge that you consult with local poultry experts, as they may be aware or resistance problems, and be able to recommend specific treatments. You could consult an avian/poultry-experienced veterinarian ASAP for complete examination, diagnosis and appropriate treatment. Checkhttp://chickenvet.co.uk/http://poultrykeeper.com/poultry-vets-uk/poultry-veterinary-practices-services-uk/
The expense for this is going to be a lot less than inefficient, ineffective, dangerous treatments, guesswork, and loss of the flock; not to mention possible implications to human consumption of tainted eggs.
Because you have others and presumably use the eggs, it is important to have a solid diagnosis and treatment safe for egg consumption. You can check with the closest Ag university with a poultry department, or with the closest vet school for a local referral.